1. Field of the Invention
The present invention relates to a GPS receiver, and more particularly to a compact GPS receiver which has a GPS antenna sharing a ground plane with an EMI shield of an electric circuit which processes satellite signals.
2. Description of the Prior Art
A GPS receiver is basically composed of an antenna unit for receiving high frequency satellite signals and a processing unit for processing the signals to compute positional coordinates of the GPS receiver. The processing unit includes an analog circuit which is responsible for amplifying the HF satellite signal, reducing the HF signal to an intermediate frequency signal, and converting the signal into a digital signal. Also included in the processing unit is a digital circuit which processes the digital signal to compute the positional coordinates of the GPS receiver. Since the digital circuit handles the digital signal, it is a source of developing a noise which interferes with the antenna and impedes the antenna gain. In order to avoid the interference, the digital circuit has to be surrounded by an EMI (electromagnetic interference) shield. On the other hand, the antenna unit is required to have increased area of ground plane for improving the antenna gain. However, in a prior GPS receiver in which the antenna unit is provided separately from the processing unit, it is difficult to make the whole assembly of the GPS receiver compact enough to be easily carried on, while affording the increased antenna""s ground plane for improved antenna gain in addition to providing the EMI shield of sufficient dimensions for successfully shielding the digital circuit.
In view of the above insufficiency, the present invention has been accomplished to provide a GPS receiver which can be made compact to be sufficiently portable. The GPS receiver in accordance with the present invention includes a GPS antenna composed of a patch and a ground plane to receive high frequency satellite signals, an analog circuit which processes the high frequency satellite signals into a digital signal, a digital circuit which processes the digital signal to compute positional coordinates of the GPS antenna, and an interface connector which outputs the positional coordinates to an external device. A substrate is provided to support the GPS antenna, the analog circuit, the digital circuit, and the interface connector. An EMI (electromagnetic interference) shield is provided to shield at least the digital circuit to block a noise developing at the digital circuit from interfering with the GPS antenna. The characterizing feature of the present invention resides in that GPS antenna is mounted on the EMI shield to share the ground plane with the EMI shield, while keeping the patch insulated electrically from the EMI shield. Thus, the GPS receiver can be made compact by making the use of the EMI shield also as the ground plane of the antenna, while assuring to block the noise from interfering with the antenna and affording sufficient antenna gain with the increased area of the ground plane.
In a preferred embodiment, the substrate comprises a single double-sided circuit board having a top mount surface and a bottom mount surface. The digital circuit is formed by a plurality of components which are mounted partly on the top mount surface and partly on the bottom mount surface. The EMI shield is composed of a top cover fitted over the top mount surface and a bottom cover fitted over the bottom mount surface. The top cover and the bottom cover are cooperative to surround the components forming the digital circuit, thereby shielding the digital circuit completely. Most preferably, the top cover and the bottom cover are cooperative to surround also components forming the analog circuit for blocking any possible noise developed at the analog circuit.
The interface connector may be a universal serial bus (USB) connector for connection with a host computer. A universal serial bus (USB) controller for the USB connector is also included in the digital circuit shielded by the EMI shield so as not to radiate undesired noise towards the antenna.
The interface connector is surrounded by a connector shield which has a top end portion projecting above a top plane of the EMI shield. The top portion is electrically connected to the top plane so as to have an electrical potential substantially equal to the top plane, i.e., the ground plane of the EMI shield for assuring a stable antenna characteristic free from the interface connector. In order to assure the reliable and easy electrical coupling of the EMI shield to the connector shield, the EMI shield is formed with a lug or lugs which project above the top plane of the EMI shield and come into surface contact with the top end portion of the connector shield
Preferably, the circuit board has a ground conductor embedded between the top mount surface and the bottom mount surface. The ground conductor is electrically connected to a plurality of pads arranged around a circumference of the circuit board. The EMI shield has a periphery corresponding to the circumference of the circuit board and has a plurality of anchors which are arranged around the periphery and bonded to the pads of the circuit board, respectively. The anchors are spaced from each other by a distance of one-fourth (xc2xc) or less of a wavelength (xcex) of the GPS signal. The distance is selected in order to block the noise from escaping through a gap between the circuit board and the EMI shield. Thus, the antenna can be completely kept intact from the noise developed at the digital and the analog circuits.
The top plane or the ground plane of the EMI shield is preferably configured into a regular polygon having four sides or more with the patch being mounted on the top plane at a location offset from the center of the regular polygon. This offset arrangement is advantageous for minimizing a loss in antenna gain when the GPS receiver is in use to be placed on a metal base. In this condition, the creepage distance from the patch to the metal base is an important factor with regard to the loss of the antenna gain. In principle, when the creepage distance from the patch to the metal base becomes nearly equal to one-fourth (xc2xc) of the wavelength (xcex) of the GPS signal, the antenna will suffer from a certain loss in the antenna gain. Therefore, as the number of such creepage distances of (xcex/4) measured from various edge portions of the patch to the metal base increases, there will be a considerably increased loss in the antenna gain. This can be avoided by the offset arrangement of the patch on the ground plane of the regular polygon. That is, the patch give different creepage distances extending from various edge portions of the patch to the metal base through corresponding edges of the regular polygon and therefore can exclude a possibility where nearly all of the creepage distances would be xcex/4. For this reason, it is possible with the offset arrangement of the patch to reduce the loss in the antenna gain when. the GPS antenna is placed on the metal base.
In this connection, a radome covering the GPS antenna is designed to have a bottom surface which is spaced from the patch by a distance longer than a critical distance below which a substantial loss in antenna gain appears.
Further, the EMI shield may be formed with a positioning structure for accurately positioning the patch on the top plane of the EMI shield, and therefore assuring reliable antenna characteristics as intended.
Also, when a feed cable is utilized to connect the patch to the analog circuit, the EMI shield may be formed with a retainer for retaining the feed cable in a fixed position for keeping the antenna characteristics as intended.
In another embodiment, a battery is incorporated in the GPS receiver to back up data stored in the memory included in the digital circuit. The battery is mounted on the EMI shield with a negative electrode of the battery being directly connected to the EMI shield, thereby simplifying a structure of connecting the battery in circuit.
Further, when the patch is connected to the analog circuit by a feed pin depending from the patch, the patch is preferably mounted at a located immediately upwardly of the analog circuit on the circuit board to minimize a connection path of the patch antenna to the analog circuit.